Existing printing presses for corrugated paper materials (such as cardboard boxes) require the use of separate printing cylinders if more than one color of ink is to be printed on the paper material. For multi-color printing, each cylinder is equipped with a separate printing die corresponding to a particular color. In a conventional multi-unit printing press, the multiple printing cylinders are collectively driven by a single primary motor located, for example, at the die cutting area of the printing press. A line shaft, chain, or gear assembly is operably driven by the primary motor and mechanically coupled to each of the printing cylinders for rotation to conduct the printing operation.
A significant aspect of multi-color printing is the importance of achieving and maintaining precise print registration among each of the multiple printing cylinders as the printing operation is conducted. This precise registration is needed to obtain proper alignment of the multi-color ink patterns on the paper material and avoid overlap or smearing of the colored ink patterns.
The above-described conventional multi-unit printing presses are capable of obtaining the desired precise registration. However, conventional printing presses encounter certain limitations that negatively impact on the ability to continuously and consistently maintain precise registration of the multiple printing units. For example, because the multiple printing cylinders on conventional printing presses are driven via mechanical coupling in the form of a line shaft or similar mechanism in conjunction with gears, these mechanical parts wear over time resulting in degradation of the print registration.
Conventional printing presses also experience problems with print registration when the printing presses require maintenance or repair. Service of the printing cylinders is frequently required to correct problems or for routine maintenance such as cleaning. When service is required, an electric compensator is used to slowly rotate the printing cylinder in either direction to position the cylinder to the area requiring service. Each printing unit on the printing press has an associated manual or electric compensator. After service, the cylinder must be returned as closely as possible to its previous position in an effort to maintain the registration among the cylinders existing before servicing. However, the use of compensators is a relatively inaccurate process for returning the cylinders to proper registration. Consequently, several test runs following service must be performed to return the cylinders to precise registration. This results in waste of both time and materials.
Electronic control systems have been developed for controlling the operation of multi-unit printing presses. For example, U.S. Pat. No. 4,527,788 to Masuda discloses a printer-slotter which has a plurality of rotary members and cylinders each driven independently by a DC servomotor. Each rotary member has a zero-point sensor for detecting a zero-point mark located on the outer periphery of the rotary member. The zero-point mark is detected by the sensor and is used to set the initial phase of each rotary member. A common reference signal is supplied to controllers for all of the units so that the speed and phase of the rotary members of all the units will be controlled according to the reference signal. Thus, the phase relationship between the separate units is maintained and the speed of the servomotors is maintained at the reference speed.
In U.S. Pat. No. 4,437,403 to Greiner, an automatic control system for adjusting register of printing plates in a multi-color printing press is disclosed. In this patent, register marks are copied on the printing plates when the plates are manufactured. Photoelectric scanners sense the register marks on the printing plates to determine the relative positions of the printing plates. The relative positions are compared and adjusted with servomotors so that all of the printing cylinders are in register with one another. The position of the printing plate having the least deviation from the corresponding zero position for a plate cylinder is chosen as the reference position to which the positions of the other printing plates are compared. The goal of this patent is to automatically align the plates on the plate cylinders in exact registration before printing starts to avoid preparation time and waste.
An important drawback of the above-described control systems is their significant expense relative to conventional printing presses. Because these control systems utilize DC motors and/or servomotors and DC drives, the expense of these components for application on a multi-unit printing press is considerable.